The simplified message service interface (SMSI) protocol, also known as the simplified message desk interface (SMDI) protocol, is a standard protocol for data communications between telecommunications switches, such as telephony central office switches and PBXs, and adjunct processors, such as voice message systems. One of the principal advantages of the SMSI protocol over other protocols is that it is very simple and easy to implement-particularly, its format is very simple and effective. However, a significant disadvantage of the SMSI protocol is that it provides only very limited functionality: the protocol defines only three message types, one of which is used by the switch to provide information about a call to an adjunct processor, a second one of which is used by the adjunct processor to activate or deactivate a message-waiting indicator on the switch, and the third one of which is used by the switch to indicate to the adjunct processor a failure of a message of the second type.
A popular feature of switches and electronic message systems is the leave-word-calling (LWC) feature. It allows a caller to leave a call-back message for a called party by dialing the called party's telephone number and pressing the LWC feature button on the caller's telephone set. The called party is then informed (for example, by means of a message-waiting indicator) that he or she has received a message. When the called party retrieves the message, it informs him or her of the calling party's telephone number and optionally of other information such as the calling party's name and the time of the call. Unfortunately, the SMSI protocol does not include functionality to implement the LWC feature.
In order to provide functionality suitable for implementing features such as the LWC feature, some telecommunication switches use protocols to communicate with the outside world that are functionally much richer than the SMSI protocol. Unfortunately, these protocols are also much more complex and difficult to implement, and so have not been widely adopted by adjunct processors. Consequently, protocol converters are used to interface such adjunct processors to such switches. In such cases, the protocol converters translate messages between the SMSI protocol used by the adjunct processors and the protocols used by the switches. Messages of the non-SMSI protocols that do not have counterparts in the SMSI message set are ignored and dropped by the converters. Consequently, functionality in addition to that provided by the SMSI protocol cannot be implemented in this manner, and the additional functionality of the non-SMSI protocols is wasted.
Furthermore, in many communication systems, and particularly in public telecommunication systems, it is common to have extensive maintenance facilities that serve to test the systems, to detect errors in the systems, and to recover the systems from errors. The maintenance facilities typically rely on extensive communications between various portions of the telecommunication system. Unfortunately, however, the SMSI protocol does not include functionality to implement maintanance communications, and thereby severely inhibits both maintenance of the link that connects the adjunct processor with the protocol converter and the integration of the maintenance of the adjunct processor and the protocol converter.